Reaction wheels provide rotating momentum to position and orient orbiting spacecraft. These stabilizing devices contain a very large and heavy wheel and an electric motor to rotate the wheel at a selected speed through motor control electronics to achieve a desired momentum C force opposing spacecraft motion in one plane. A typical spacecraft can have three reaction wheels to stabilize the craft around three axes and provide for craft maneuvers by changing the speed of the wheels.
The motor and wheel are supported on bearings in a reaction wheel housing which must have sufficient strength and rigidity to support the considerable weight of the motor and wheel and the forces that are exerted by the wheel on the craft when the craft moves. The control and drive electronics are sometimes contained within the housing with associated power and control wiring routed and bundled inside the housing, making removal of the electronics difficult. The electronics need to be tested in an environment simulating space conditions, that is, in a vacuum, and this has meant that the electronics are tested in a vacuum chamber prior to insertion into the housing. To avoid damaging the electronics following such testing, the electronics are more often installed outside the wheel housing--rather than risk damaging the tested electronics from the manipulations required to install them inside a small compartment in the housing.